Continuity mistake: After Marge says she will marry Homer, at the beginning of the scene, the car is facing the power plant, and at the end, it is facing away from the plant. (00:07:45)
Continuity mistake: In the episode which recalls how Homer and Marge got married, Homer writes a note to Marge on a piece of paper entitled 'From The Pen Of Patty' he SCRIBBLES out Patty's name and writes his own, but when Marge reads the note, 'Patty' only has a single straight line through it. The word "HOMER" also changes. (00:14:30)
Other mistake: In the shot before Marge says she will marry Homer, the right side of the screen is cut off a little. (00:08:25)
Other mistake: When Marge sees the letter from Homer, she says "You won't see me again until I'm a man." On the paper though, it reads 'But you will not see me-' and the sentence is cut off, and the rest of the paper is blank. (00:16:10)
Continuity mistake: When the Repo Man takes Marge's ring, we see the ring on her left hand. It wasn't visible in the shot before. (00:14:40)
Audio problem: In the car, when Homer accidentally kicks the horn twice, the second noise goes longer than it's supposed to. (00:08:10)
Continuity mistake: At the beginning, when Homer and Marge are discussing the pregnancy test, the pattern on the bathroom walls, disappears and reappears, and in one shot, there's pattern on one side of the wall, but not on the other.
Continuity mistake: When Homer is making a note for Marge, he scribbles out Patty's name and writes his own. When he writes it, the "signature" is neat, but when Marge reads it, it's sloppy. (00:14:30)
Continuity mistake: When Marge's sisters start smoking in the flashback, there is only one ledge on the wall behind them. In the next shot there are three ledges and a clock. (00:04:05)
Continuity mistake: As Homer and Marge are singing in the car, there is one exterior shot of the car, showing it is yellow, but the rest of the scene it is green. (00:05:55)
Continuity mistake: As Homer and Marge are about to get married, Homer disappears for one shot. (00:10:20)
Continuity mistake: When Homer is writing a note to Marge, he scribbles out Patty's name and writes his own name directly beneath it. When Marge reads it, Homer's name is under Patty's, but slightly to the right. (00:14:30)
Other mistake: When Homer talks to Bart and Lisa through the window, he is standing almost straight up, but considering he is right in front of the TV in the next shot, he would've had to crouch down a little. (00:03:50)
Continuity mistake: When Homer sees that Bart and Lisa aren't listening to his story, he looks around the living room while sitting on the couch. He appears beside the window in the next shot. (00:03:30)
Other mistake: When Homer leaves the window, the TV is in a different area of the living room. (00:03:50)
Continuity mistake: In the flashback, at the Aztec Theater, the person in front of Marge and Homer changes. (00:04:40)
Continuity mistake: Before Homer and Marge get married, they are taking more steps then they are supposed to, as you can see the interior of the wedding chapel before this scene. (00:10:10)
Continuity mistake: When Marge comes into the living room at the end, the chinese checkers game that Bart and Lisa were just playing is gone. (00:21:15)
Audio problem: When the old lady is playing the keyboard at Homer and Marge's wedding, her hands don't match the music.
Continuity mistake: When Marge is opening the letter and pulls the money out, the shot changes and the money is no longer in her hand. (00:16:20)
Answer: I'm not sure if this is the one you're thinking of, but an episode of "Ray Bradbury Theatre," called "A Sound of Thunder," dealt with a similar matter: a group of hunters travel back in time to hunt dinosaurs, only to find things have changed when they get back because someone stepped on a butterfly.
Xofer